The duck's foot alignment is better than the foot alignment in the previous photo.
As your center of mass passes over your foot, your weight is putting stress on the inside of the foot near the big toe, like this photo.
Foot mechanics—left foot aligned and right foot creating stress riser in foot.
Your foot responds by building up more of a "foundation" to support your weight, and the result is a bunion. Another factor in bunions, of course, is tight shoes, but don't discount the effect improper loading of the joint has on the body structure. It doesn't stop at bunions though, because your knees, hips, and spine also have to adapt to the inappropriate foot position. Your joints won't load correctly when your bones aren't aligned.